Adsorption And Interfacial Dilational Viscoelastic Properties Of Quaternary Ammonium Gemini Surfactants In Oil/Water System

Gemini surfactants are generally made up of two hydrocarbon chains and twohydrophilic head groups covalently connected by a spacer at the level of, or near tohydrophilic head groups. The spacer is the most important feature of Geminisurfactants. The chemical structure and length of the spacer strongly affects theself-assembly of Gemini surfactant both at the interface and in the bulk solution. Inthis thesis, we systematically investigated the adsorption and interfacial dilationalviscoelastic properties of a series of cationic Gemini surfactants alkanediyl-bis(dimethyldodecylammonium bromide), referred below as12-s-12, in the oil/watersystem. The effects of molecular structure and spacer length on the interfacialadsorption have been revealed. The conclusion obtained is instructive for constructionof highly stable interfacial films. The main results in this thesis are summarized asfollows:1. The adsorption and micellization of12-s-12in the isopropyl myristate(IPM)/water system have been investigated using interfacial tension measurements.The results showed that the12-s-12has much stronger ability of adsorption at theIPM/water interface and micellization in the water phase in comparison with thecorresponding monomer, C12TABr. Due to the folding of the long spacer chain, theadsorption of12-s-12with s≥12at the IPM/water interface undergoes two differentstages. At the beginning, the12-s-12adsorbs on the IPM/water interface almostwithout any resistance. At the second stage, with further increasing the concentration,more12-s-12molecules crowd into the adsorption layer, leading to strong bending ofthe spacer chain in order to make the room to admit more molecules at the interface.The addition of salt promotes their adsorption at the IPM/water interface and resultsin the formation of interfacial aggregates. The measurements of dilational elasticityfor the adsorption films supported the above mechanism.2. Interfacial dilational rheological measurements were used to investigate theviscoelasticity of the adsorption films of12-s-12(s=2~18) at different concentrations in the IPM/water system. The results showed that the interfacial elasticity graduallyincreases with increasing frequency at a certain concentration and finally reaches thelimiting dilational elasticity, whereas the interfacial viscosity decreases sharply withincreasing frequency. Both interfacial elasticity and viscosity runs through amaximum with increasing surfactant concentration.3. The experimental data was analyzed theoretically with a popular model (LVTmodel), from which the limiting elasticity ε0,fitand the characteristic frequency ofmolecular exchange ω0,fitcan be obtained. The molecular structures of Geminisurfactants strongly affect the stability of the adsorption film. Over the range of s=2tos=8, high limiting dilational elasticity corresponds to a moderate length of the spacerchain(s=3-4), which means that the films of12-3-12and12-4-12may be more stablethan others.4. The12-s-12with long spacer chain can afford greater value of ε0,fitcomparedwith that with short spacer. The12-s-12(s≥12) bends its long spacer chain toward theoil-side. This not only makes the molecules adsorbed tightly at the interface but alsobrings about larger elasticity of the adsorption film due to bending-able long spacerchain. This suggests that the12-s-12with long spacer may be the better choice whendeveloping this series of Gemini surfactants as emulsifiers.5. The adsorption and viscoelasticity of12-s-12in the hexane/water system havealso been investigated using interfacial tension and interfacial dilational rheologicalmeasurements. The results showed that the nature of the oil phase affects thedilational viscoelstic property of the adsorption film to some extent.12-s-12at thehexane/water interface owns higher efficiency in reducing the interfacial tensioncompared with those at the IPM/water interface.